Art of mounting piezoelectric crystals



Oct. 14, 1941. s. A. BoKovoY 2,258,967

ART OF MOUNTING PIEZOELECTRIC CRYSTALS Filed June 29, 1940 /V F561. E 71/ Srmentor Jamuel.olovoy (ttorneg Patented Oct. 14, 1941 ART F DlNTING PIEZOELECTRIC CRYSTALS Samuel A...Eokovoy, Audubon, N. J., assignent-o .Radio Corporation of America, a corporation of Delaware lApplieation June 29, 1940;'Serial No. 343,140

(Cl. Fil-327') 6 Claims.

LThis invention relates to ltheart of mounting piezo-electric crystals.

It is appreciated bythoseskiiled inthe art to whichA this invention appertains that optimum performance of a crystal-controlled circuit is only achieved when the electrical (as distinguished from piezoelectrical) characteristicsof the crystal and itsmountmatchthe said circuit. In matching a crystal to a circuit, the technician has regard to the physical dimensions of the crystal because size is the principal factor determining the electrical resistance and inductance of the crystal. Where, as is usually the case, one mustconsider the capacitance contributed to the circuit bythe crystal and its electrodes, it is customary to make the opposed electrode platesor films of a size and spacing calculated to provide the -desired value of capacitance. In this latter case, diiculties arise where it is necessary or desirable to apply a clamping force to the crystal principally because it has heretofore been thought necessary to leave a large uncovered (and hence non-capacitive) area, soL- ly for clamping purposes, on each electrode face of the crystal. Instances have arisen, particularly in connection with crystal resonators, where the clamping area embraced so much of the surface area of the crystal that the remaining or capacitive area. of the crystal faces was not great enough to establish electrical properties of a value calculated to match the filter circuit for which the crystal was cut.

Accordingly, the principal object of the present invention is to provide an improved clamped crystal assembly which shall exhibit substantially any desired value of capacitance, or other electrical constant or characteristic, within limits determined primarily by the overall dimensions of the crystal.

Another object of the invention is to provide a clamped crystal assembly wherein each clamp exerts its clamping force upon two oppositely located electrode coating areas on the crystal, whereby that part of the coating which is presented to the clamps has a greater contact area and is less subject to abrasion and wear than in prior art mountings wherein each clamp contacts the electrode coating at but a single point.

Another object of the invention is to provide e.'

a plated crystal capable of being clamped without danger of a short circuit.

Other objects and advantages, together with certain details of construction, will be apparent,

reference to;the following `specification and to the accompanying drawing, wherein:

Figure 1 is a view in perspective cfa piezoelectric element which is provided in accordance Withthe invention with a pair of electrodes of a design calculated to provide a desired high value of capacitance.

Figures 2.and3 are side elevational viewsof a crystal showing other'arrangementsv of electrodes.

Figure 4 is an exploded view of a holder showing in which the crystal and its electrodes vmay be mountedy and Figure 5 is a fragmentary view of the device of Fig. 4 assembled and showing certain details of a clamping element therein.

The foregoing and other objects are achieved in accordance with the invention by providing a piezo-electric crystal with'a pair of electrodes each of which partially covers two of the major or electrode faces of the crystal and, preferably, one or more of its minor faces. The necessary electrical connections to the separate electrodes preferably comprise a pair of spring clamps the jaws of which embrace both sides of the covered end zones of the crystal.

In Fig. l, P designates a crystal element or plate .possessing piezo-electric properties and which may be constituted of quartz, tourmaline or Rochelle salts. The crystal P is provided with a pair of electrodes El and E2 which preferably comprise adherent foil-like or integral film-like coatings of metal. Each electrode partially-covers the two electrode or major faces MI, M2,

" and three of the minor faces N of the crystal.

The capacitance between the major surfaces M of the crystal P is enhanced in this case by the capacitance, indicated by the phantom oapacitors C, which obtains between the edges of the electrodes E along both of the major faces MI, M2 and along the two side minor faces NI, N2. The eifective capacitance may be still further augmented by making the opposed edges of the electrodes of undulate or serrated contour, as

l indicated at S.

When, as in the instant case the electrodes E comprise a pair of film-like metallic coatings, the metal may be applied to the crystal by thermal evaporation, by chemical deposition or by the so-called Rochelle salt process. (As to these three processes, see Procedures in Experimental Physics, by Strong, Prentice-Hall, Inc., 1938.) In any case, during the deposition of the metal the marginal space between the electrodes and the invention will be best understood by 5'7 may be covered by a mask of the desired pattern and formed, by way of example, of nitrocellulose la-cquer or other substance which may be readily removed (as with acetone) without injury to the electrode coatings. In some cases, as where the exact ultimately desired value of capacitance is not known in advance, masking may be dispensed with and the entire surface of the crystal first covered with the metal. The metal may then be erased in a desired pattern as by means of a wooden stylus the 4point of which has been dipped in nitric acid.

Where the value of capacitance desired is less than that which may be achieved with electrodes of the pattern of Fig. 1 (but is nevertheless greater than can be achieved with a pair of electrodes which simply cover each of the major faces of the crystal) the electrodes may assume a pattern similar to that shown in Fig. 2. In Fig. 2 the electrodes El and E2 are shown as each partially covering the major faces MI, M2, one entire minor end-surface NI or N3, and but a small area on the side minor faces N2 or N4 of the crystal.

Referring to Fig. 3. Where a still smaller value of capacitance is desired, the coating on the side minor faces N2, N4 of the crystal C may be entirely erased or omitted in which case the effective capacitance of the assembly is that which obtains between the major faces MI, M2 and between the opposed edges of the electrodes El and E2, on said major faces.

Referring now to Figs. 4 and 5. The necessary electrical connections to the electrodes El and VE2 preferably comprise a pair of spring clamps J I and J 2, each having jaws which embrace an end zone of the crystal. Thus each clamp is in contact with its electrode at two points, i, e., on the top and bottom or major surfaces, MI, M2 of the crystal, (instead of only on one conductive side or area, as in the prior art). This not only ensures an electrically better connection but also one less likely to be broken as by abrasion caused by the normal vibratile movement of the crystal in contact with its clamps.

The clamped crystal assembly of the invention may be enclosed in a hermetically sealed casing H, if desired, for example in the manner described in copending application Seria1 No. 276,677 to H. W. N. Hawk, filed May 31, 1939. In this case, referring particularly to Fig. 5, a jaw of each clamp` J may be provided with an extended lip L which bears against the Wall of the cavity of the casing to maintain the clamping force upon the crystal at a fixed value and, at the same time preventing undesired movement of the crystal within the cavity.

Various modifications of the invention including variations in the electrode patterns will suggest themselves to those skilled in the art in the light of the instant disclosure. Accordingly, it is to be understood that the foregoing is to be interpreted as illustrative and not in a limiting sense except as required by the prior art and by the'spirit of the appended claims.

What is claimed is:

1. In combination, a rectangular piezo-electric crystal having major and minor faces, a pair of electrodes each partially covering two major and three minor faces of said crystal and means for applying a clamping force to said crystal through its partially covered major faces.

2. The invention as set forth in claim 1 and wherein the edges of said electrodes which lie on a common minor face of said crystal are presented in useful capacitive relation to each other.

3. In combination, a piezo-electric crystal having major and minor faces, a pair of electrodes each partially covering two major and at least one minor face of said crystal, and means for applying a clamping force to said crystal in a direction substantially normal to the plane of Vits major faces and contiguous the areas on said major faces which are covered by the same electrode.

4. In combination, a rectangular piezo-electric element, a pair of electrodes each covering an end zone of two major faces of said crystal, and means for applying a clamping force to said crystal contiguous said end zones in a direction substantially normal to its said major faces.

5. In combination, a piezo-electric crystal, a pair of electrodes each partially covering two electrode faces of said crystal, the covered areas of said electrode faces embracing the opposite end Zones of each of said crystal faces, and a pair of electrode leads each secured to said'crystal contiguous said covered end zones.

6. In combination, a piezo-electric crystal having major and minor faces and a pair of'electrodes each partially covering two major andat least one minor face of said crystal, the edges of said electrodes which lie on a common face of said crystal being of non-linear contour and presented in useful capacitive relation to each other.

SAMUEL A. BOKOVOY.

DISCLAIMER 2,258,967.Samuel A. Bolcovoy, Audubon, N. J. ART 0F MOUNTING PIEZOELEC- TRIO CRYSTALS. Patent dated October 14, 1941. Disclaimer led September 30, 1942, by the assignee, Radio Corporation of America. Hereby enters this disclaimer to claims 1, 3, 4, and 5.

[Official Gazette November S, 1942.]

D l S C L A l M E R 2,258,967.Samuel A. Bokoooy, Audubon, N. J. ART 0F MOUNTING PIEzoELEc- TRIC CRYSTALS. Patent dated October 14, 1941. Disclaimer filed September 30, 1942, by the assignee, Radio Corporation of America. Hereby enters this disclaimer to claims 1, 3, 4, and 5.

[Oficial Gazette November 3, 1.942.] 

